1. Field of the Invention
The present invention relates to a data reproduction device, and in particular to equalization of a digital data reproduction signal.
2. Description of the Related Art
There is known a technique for reproducing digital image and speech signals recorded on a magnetic tape or the like. A device employing this technique equalizes a reproduced signal using an equalizer so that deterioration of the signal as it passes through the recording and reproduction system, or variation among magnetic tapes, tape types, or magnetic heads are compensated for so that errors can be reduced.
FIG. 14 is a block diagram showing a structure of a conventional reproduction device having an equalizer. A reproduction circuit 103 comprises a rotating magnetic head and reproduces information, such as an image signal, a speech signal, and a sub-code, which are recorded on a track of a magnetic tape 101, to output as an analogue signal. An amplifier 105 amplifies the reproduced signal sent from the rotating head before outputting to an equalizer 107.
The equalizer 107 comprises a high region enhancement filter, an all-pass filter for adjusting a group delay in a low region (that is, a low region group delay), and an all-pass filter for adjusting a group delay in a high region (that is, a high region group delay). The high region enhancement filter compensates for deterioration of a high region component of a signal sent from the amplifier 105 before outputting to the low region group delay adjustment all-pass filter. The low region group delay adjustment all-pass filter adjusts a group delay in a low region of the analogue reproduced signal so that dipulse characteristic of the magnetic tape can be compensated for, before output to the high region group delay adjustment all-pass filter. The high region group delay adjustment all-pass filter adjusts a group delay in the high region of the analogue reproduced signal, and then applies integration to the signal before outputting to an A/D 109.
The A/D 109 converts the analogue reproduced signal sent from the equalizer 107 into a digital signal before outputting to an FIR (Finite Impulse Response) filter 111 and a PLL 113. In the above, the A/D 109 samples the analogue reproduced signal in response to a clock supplied from the PLL 113.
The FIR filter 111 comprises a plurality of latches and coefficient units, and adjusts group delay characteristic of a signal before output to a PR4 decoder 115. The PR4 decoder 115 demodulates a digital signal subjected to PR4 pre-coding before outputting to an evaluation circuit 117 and a Viterbi decoder 121. The evaluation circuit 117 evaluates the characteristic of the equalizer 107, and outputs the evaluation result to an equalizer control circuit 119 to adjust the characteristic of the equalizer 107.
The Viterbi decoder 121 detects a digital signal of one-sample-one-bit out of the output from the PR4 decoder 115 according to the Viterbi algorithm before outputting to a signal processing circuit 123. The signal processing circuit 123 obtains an reproduced image signal and a reproduced speech signal based on the digital signal obtained by the Viterbi decoder 121 to output via an output terminal 125. Japanese Patent Laid-open Publication No. 20001-209902 discloses the above-described data reproduction device.
In the above, variation can be accommodated to some extent when an analogue reproduced signal obtained by the reproduction circuit 103 is equalized by the equalizer 107. However, there exist the limits in improving the error reduction capability and/or characteristic variation accommodating capability. In particular, as a variety of magnetic tapes and heads have recently become available and, accordingly, characteristic variation become more diversified, demands arise for cost reduction, more stabilized performance, and user demand respondable flexibility. Particularly, an equalizer having variably, flexibly, and accurately adjustable equalizing characteristic is desired.